The development of very large scale integrated circuits (VLSI) has substantially increased the circuit content of logic cards used in data processing systems. With more circuits, there is an attendant need for more signal connections to the card. The problem was addressed in the early stages of development of the so-called "personal computer" by connecting each card to a bus structure. This bus allowed the circuits on a card to communicate with circuits on another card. The various engineering trade-offs involving circuit cost and performance led to the adoption of a parallel bus architecture. Unfortunately, the parallel arrangement, while providing good performance, also carries the need for a large number of connections. Since there is a practical limit to the number of connections which can be economically made to a single logic card, one solution involves the use of a high speed serial data bus. This reduces the number of connections required, but introduces higher cost.
The problem is further complicated by the fact that many data processing systems have variable configurations. That is, they may be shipped from the manufacturer to the user with a given configuration including certain logic cards. The customer may then add other logic cards to enable the system to better satisfy his particular data processing requirements. This means that any bus structure must accommodate the installation of logic cards by an unskilled person, which rules out the use of otherwise viable approaches. For example, high speed data transfer is achievable with optical devices connected by fiber optic cables, but these require the use of sophisticated connectors which cannot be reliably installed by unskilled persons. Alternatively, an optical data bus can be implemented with lenses instead of fiber optic cables, but these require precise alignment, which may not be possible by unskilled installers, and are much more expensive to implement.
Despite these shortcomings, the development of low cost semiconductor lasers and high speed photosensitive devices such as PIN diodes has made optical transmission of data competitive with hard wired systems. The great band width which may be achieved with optical transmission systems, their immunity to electrical noise and absence of cross talk and spurious emissions have led to widespread adoption of such systems in the data processing and communications environments.
U.S. Pat. No. 4,063,083 to Cathey and Smith describes a data communications system for transfer of data between printed circuit logic cards in a data processing system. LED transmitters are positioned to illuminate receptors and apertures in a fashion to provide a parallel data bus connection with all logic cards in a system. The bus is implemented with an appendage to the conventional logic card. The appendage carries the requisite LED's, lenses, photoreceptors and apertures. The system of the patent provides a parallel data bus. Although the system works in free space, and therefore does not require fiber optic elements, it requires lenses, a high degree of precision in alignment of the logic cards, and is not adapted for use within the confines of a conventional logic card. In contrast, the system of this invention is directed to a serial data bus which is implemented without the use of lenses and can be implemented within the confines of a conventional logic card without the need for precise alignment.
U.S. Pat. No. 4,161,650 to Cauette et al. shows a bi-directional fiber optic data communications link. LEDs are used as transmitters and photo diodes as detectors. Fiber optic elements in the interconnect cable carry a serial data signal, clock, and control signals. A conventional electrical conductor, packaged with the fiber optic elements, carries power to the remote station. The system of this invention differs since the optical bus is implemented in free space without fiber optic elements.
U.S. Pat. No. 4,358,858 to Tamura et al. is directed to an optical data communications system in which the transmitters and receivers of all stations are physically arranged to transmit light to, and receive light from, a common mirror. In this fashion all stations can communicate with all other stations without the need for fiber optic elements. In contrast, the optical bus system of this invention is implemented in loop fashion, where each station directly communicates with only the stations on either side. Further, the system of this invention does not require optical alignment beyond the precision afforded by the usual card socket and frame used in conventional logic card support.
U.S. Pat. No. 4,494,185 to Gunderson et al. relates to an optical implementation of broadcast packet switching in which a pair of fiber optic lines connect each station to a star coupler, which provides the bidirectional capability by connecting one of the fiber optic lines to the other. In the system of the invention claimed herein, transmission occurs in free space, without the use of fiber optic elements, and each card can communicate with only the card on either side.
U.S. Pat. No. 4,527,285 to Kekas el al. describes a terminal packaging technique which provides space dedicated to the use of optical transmission between modular units of the terminal. The patent does not describe the optical data communications system.
U.S. Pat. No. 4,566,134 to Harbour et al. shows a system for interconnecting modular terminals of the type described in U.S. Pat. No. 4,527,285. Fiber optic cables are coupled to the optical signal which exists in the dedicated space. In turn, the coupled fiber optic cables lead to the remote terminal devices. The claimed invention does not require the use of fiber optic cables, and is directed to a loop arrangement, not the broadcast system of the reference.
U.S. Pat. No. 4,499,608 to Broockman et al. describes a terminal having an optical chamber in which each modular unit of the terminal communicates directly with the other units by reflecting the signals off a mirror. The claimed invention differs in that each unit communicates only with the units on either side to provide a loop arrangement rather than the broadcast configuration of the reference.
U.S. Pat. No. 4,449,206 to Tokitsu et al. is directed to a portable semiconductor memory system in which address and data information is transmitted between the memory and a data processing system by an optical data link. The claimed invention is distinguished by the existence of a logic card which can communicate with the cards on either side without the need for lenses or optical fibers.
U.S. Pat. No. 4,393,515 to de Neumann relates to a multi-processor arrangement in which all the processors communicate with all other processors by means of optical signals transmitted and received through a device which includes a conical prism. The claimed invention differs since each logic card can communicate only with the cards on either side and no prisms or lenses are required.
U.S Pat. No. 4,499,607 to Higgins shows a semiconductor chip assembly in which the chip-to-chip signal transmission is achieved with optical transducers located at the edge faces of the chips. The patent does not show logic cards with optical transducers on opposite sides thereof as claimed herein.
Japanese Pat. No. 58-139285 (A) to Kominame relates to a card which carries an integrated circuit and a reader for the card which communicates by optical means. The card-reader system of the patent does not relate to the optical data bus communication system for logic cards as claimed herein.
Japanese Pat. No. 59-126335 (A) to Kawarazuka describes an optical data bus for use in a record player. The data bus includes an optical transmission element having appropriately positioned prisms which serve to disperse the radiation in the transmission element to allow reception by a plurality of photodetecting elements. The claimed system distinguishes by the absence of such elements as dispersing prisms.
Japanese Pat. No. 59-169237 (A) to Morimoto relates to an optical communications system for a portable terminal. The system of this patent does not include an optical data communications system for the interconnection of logic cards as claimed herein.
IBM Technical Disclosure Bulletin Vol. 26, No. 4, Sept. 1983, pp 1793-1796, Balliet and Cowden, shows an electronic unit optical interconnecting system which uses a pair of conical prisms at each unit to disperse and acquire optical beams for transmission and reception. In this fashion, each unit is able to directly communicate with all other units. In contrast, the claimed system does not require the use of optical elements such as prisms and provides direct communication with only the adjacent devices.